1. Field of the Invention
The invention relates in general to the reduction of cogging torque in permanent magnet motors, and particularly to brushless permanent magnet motors.
2. Description of the Related Art
Permanent magnet motors include a stator core, which is typically made of a stack of thin, metal laminations. The laminations are usually round, with a central opening. The stator core thus is generally cylindrical in shape, with a cavity extending lengthwise about its central axis. In brushless permanent magnet motors, each stator lamination includes radially-extending slot openings, or notches, from the central opening that are aligned when stacked to receive stator windings, or conductors.
The stator core surrounds a rotor, typically consisting of a circular steel shaft with a number of permanent magnets fixed around the circumference of the shaft. The rotor can also comprise a stack of laminations instead of a solid steel shaft. The permanent magnets are magnetized to form a plurality of magnetic poles.
In permanent magnet motors, cogging torque is caused by the combination of two factors, the permanent magnet magneto-motive force and the variation of the air gap permeance between the stator and the rotor. Cogging torque is represented by the following formula:
Tcog(xcex8r)=xe2x88x92dW/dxcex8r=xe2x88x92(xc2xd)(MMF)2(dxcex/dxcex8r),
wherein
Tcog is the cogging torque;
W is the total energy of the field;
xcex8r is the rotor position angle;
MMF is the magnetic excitation of the permanent magnets; and
xcex is the air gap permeance.
In the design of permanent magnet machines, cogging torque is a concern because it adds unwanted harmonic components to the torque-angle curve, resulting in torque pulsation upon operation of the machine. Although net cogging torque is zero, it causes noise, power losses and inaccuracies, particularly in servo-positioning drives. Thus, reduction of the momentary cogging torque is desirable.
The reduction of cogging torque can be approached in a variety of ways. One is to reduce the rate of change of the air gap permeance. Another is to shift poles so that cogging torque produced by one pole cancels another. Finally, since the magnetic excitation of the permanent magnets is squared in the formula, a reduction in magnetic flux produced reduces cogging torque. One means disclosed by the prior art to reduce cogging torque is by shaping permanent poles using a tapered arc. Another is skewing the lamination stack of the stator. In either case, the use of discrete magnets can result in excessive cogging torque because of manufacturing tolerances in orienting the magnets on the rotor surface.
To overcome the problems with discrete magnets, it is sometimes possible to use a ring magnet mounted on the core, but the magnets of the ring must be perfectly aligned in some motors, or the cogging torque is increased. For example, in a permanent magnet brushless motor with nine slots and eight poles, the cogging torque is reduced by the cancellation effects from one magnet to another. When there is misalignment, the cogging torque increases. Thus, it is desired to create a machine design that reduces cogging torque through pole alignment without the drawbacks of present methods.
The present invention is a rotor for a permanent magnet motor comprising a rotor yoke and a permanent magnet ring mounted on the rotor yoke, the permanent magnet ring including a plurality of circumferentially spaced poles. One of the rotor yoke and the permanent magnet ring is an annular member including depressed portions along an outside peripheral edge, each depressed portion located around a midpoint between two poles. The depressed portions are shaped so the motor produces a sinusoidal flux density. In one aspect of the invention, the other of the rotor yoke and the permanent magnet ring is an annular ring.
In one aspect of the invention, the rotor yoke is skewed. In this aspect, the magnet ring preferably includes the depressed portions, but another aspect includes the depressed portions on the rotor yoke.
In another aspect of the invention, each of the plurality of poles is tapered along each depressed portion. In this aspect, the magnet ring preferably includes the depressed portions.
The rotor yoke can comprise a stack of laminations in yet another aspect of the invention.
Preferably, the permanent magnet ring is a pressed permanent magnet ring.
The depressed portions can form a variety of shapes. For example, the depressed portions can form roughly trapezoidal or ovoid depressions. Each of the depressed portions can also form an apex of a triangle. In a preferred aspect, each of the depressed portions is uniform in shape.
In yet another aspect of the invention, the magnet ring includes six poles.
The magnet ring comprises one of a rare-earth magnetic material and a ceramic magnetic material.
Another rotor of the present invention comprises a rotor yoke and a permanent magnet ring mounted on the rotor yoke, the permanent magnet ring including a plurality of circumferentially spaced poles. One of the rotor yoke and the permanent magnet ring includes a plurality of depressions along an outer peripheral edge. Each of the plurality of depressions located around a junction defined by two poles, and the plurality of depressions are shaped so the motor produces a sinusoidal flux density during operation.
In this aspect, the outer peripheral edge can be annular between the plurality of depressions. In another aspect of the invention, the other of the rotor yoke and the permanent magnet ring has an annular outer peripheral edge.
In yet another aspect of the rotor, the rotor yoke is skewed. Each of the plurality of poles can be tapered along each of the plurality of depressions.
In another aspect, each of the plurality of depressions is uniform in shape.
The present invention improves the orientation of magnet poles with respect to each other. It allows the use of tapered magnetic poles with a ring magnet and/or skew of the rotor core.